1. Field of the Invention
The present invention relates to surface acoustic wave devices having a configuration in which a surface acoustic wave filter and a surface acoustic wave resonator are interconnected. More specifically, the present invention relates to a surface acoustic wave device having a configuration in which a film having a positive frequency-temperature characteristic is disposed on a piezoelectric substrate and also relates to a method for adjusting the frequency thereof.
2. Description of the Related Art
In recent years, in conjunction with an increase in subscribers and diversification of services, systems having wide transmission and reception bands and having transmission and reception frequency bands that are close to each other have been increasing. Correspondingly, there has been great demand for a bandpass filter having a broader passband and a greater attenuation in close proximity to the passband.
Surface acoustic wave filters (herein after simply referred to as “SAW filters”) have been widely used as RF filters for portable telephones. Such a SAW filter uses a 36° to 44° Y-cut X-propagating LiTaO3 substrate having a large electromechanical coefficient to realize a broad band. However, a 36° to 44° Y-cut X-propagating LiTaO3 substrate has a frequency-temperature dependency of as much as −30 ppm/° C. to −35 ppm/° C. Thus, with a SAW filter including the substrate, a greater tolerance relative to a temperature change must be provided. Consequently, it has been difficult to increase the attenuation in close proximity to the passband.
As a method for compensating for the frequency-temperature characteristic of a 36° to 44° Y-cut X-propagating LiTaO3 substrate, Japanese Unexamined Patent Application Publication No. 2-037815 proposes a method in which an Al electrode is formed on the substrate and then an SiO2 film is further deposited thereon. In this publication, an SiO2 film having a positive temperature coefficient is formed on a 36° to 44° Y-cut X-propagating LiTaO3 substrate having a negative temperature coefficient, thereby reducing the absolute value of the temperature coefficient.
Deposition of SiO2 film, however, causes a problem in that the propagation loss of the SAW filter is increased and thus, the electromechanical coefficient is reduced.
FIG. 16 shows the frequency characteristic of an exemplary longitudinally-coupled three-IDT-type resonator filter of the related art which is formed on a 36° Y-cut X-propagating LiTaO3 substrate. In FIG. 16, the “characteristic represented on an enlarged scale” indicates a characteristic that is plotted against the large scale given at the right side of the vertical axis. FIG. 17 shows a frequency characteristic when an SiO2 film is deposited so as to have a thickness equal to 15% of the wavelength that is defined by the pitch of the electrode fingers.
As is apparent from the comparison between FIGS. 16 and 17, the passband of the filter has a significant depression at the center, since the propagation loss deteriorates due to the formation of the SiO2 film, the in-band insertion loss considerably deteriorates, and the electromechanical coefficient becomes small.
The deterioration of insertion loss due to the formation of the SiO2 film as described above becomes more significant as the center frequency of the filter is increased. Thus, at frequencies at which the filter is used as an RF filter for a portable telephone, the in-band insertion loss thereof is deteriorated to such a degree that the filter cannot be used.
In addition, since the electromechanical coefficient becomes small, it has been difficult to broaden the bandwidth of the filter. Thus, in the art of RF filters, it has been difficult to use methods intended for improving a frequency-temperature characteristic by forming a SiO2 film.